The knowledge of the contribution of carbon (C) released by growing roots to soil is essential to better understand the terrestrial C cycling and optimally manage soil organic matter in ecosystems. However, little information has been gained on quantifying the distribution of photosynthetically fixed C in the plant–soil system and its contribution to soil C over a growing season in soybean-grown Mollisols, the main soil type in Northeast China. In a pulse-chase labelling experiment, soybean plants grown in Mollisols were labelled with 13CO2 at various growth stages. More than 3/4 of fixed 13C was observed in shoots at Day 0 after labelling, and then the fixed 13C was continually exported from shoots, showing that 7.5% of 13C fixed at V4 (fourth node) and 71.1% at R6 (full seed stage) remained in shoots by the end of the growing season. The 13C recovery in roots decreased over the same period, while soil 13C was significantly increased. The allocation of 13C fixed at different growth stages to underground (roots and soil) varied at the end of the growing season, showing that 13C retained in roots and soil was 6.0 and 12.4% of the net assimilation at V4, compared with 1.4 and 2.1% of that at R6, respectively. Nodules, however, had the highest demand for C at R4 (full pod stage). The contribution of shoot C assimilation to the soil C pool was similar at the growth stages up to R5 before a sharp decrease at R6, and the cumulative contribution reached 93% at R5. Over the whole growing season, it was estimated that ~210 kg of photosynthetically fixed C per ha was accumulated in soil. This indicates that the C flow from soybean plants to soil during growth is a non-negligible source of C pool in Mollisols, and the majority of the C efflux occurs during V4–R5.
Poor growth of white lupin (Lupinus albus L.) on alkaline soils may result from its sensitivity to iron deficiency and poor nodulation. This study examined interactive effects of iron supply and high pH on the growth and nodulation of three genotypes differing in their sensitivity to iron deficiency. Three genotypes (P27486, Ultra and WTD180) were grown for 17 days in buffered solutions with Fe supply of 0.2, 2 and 20 lM. Solution pH was adjusted to 5.2, 6.5 or 7.5. Plant growth, nodulation and nutrient concentrations in plants were measured. Decreasing Fe supply decreased chlorophyll concentration in young leaves by up to 92%. Increasing pH decreased chlorophyll concentration by an average of 40% at pH 6.5 and by 47% at pH 7.5. The decrease of chlorophyll was less obvious in P27485 than in Ultra or WTD180. Shoot biomass was reduced by up to 18% by Fe deficiency, with such decrease being less for P27486. Increasing pH exacerbated the effect of Fe deficiency on shoot biomass only of Ultra. Decreasing Fe supply decreased nodule number by an average of 54%, and increasing pH decreased nodule number by 80%. P27486 formed the greatest number of nodules while WTD180 the least. P27486 had high Fe uptake and low internal requirement. Irrespective of genotype, leaf chlorosis positively correlated with cluster root formation. The results suggest that a combination of Fe deficiency and high pH impaired nodulation in L. albus, and that selection of genotypes for both tolerance of iron deficiency and good nodulation at high pH is important for a successful lupin crop on alkaline soils.
Excessive fertiliser has been commonly applied in the soybean (Glycine max (L.) Merr.) cropping system in fertile Mollisols in Northeast China. However, it is necessary to understand how reducing nitrogen (N) fertiliser application may affect plant N acquisition and remobilisation, which is associated with photosynthetic carbon (C) assimilation and seed yield. The aim of this study was to investigate the origin of plant N (i.e. derived from N2 fixation, fertiliser or soil) under two different levels of N application, and the subsequent influence on C assimilation. A pot experiment was conducted with soybean grown in a Mollisol supplied with 5 mg N kg–1 soil (N5) or 100 mg N kg–1 soil (N100). Nitrogen was applied as 19.83% of 15N atom-excess in urea before sowing, and 13CO2 labelling was performed at the R5 (initial seed-filling) stage. Plants were harvested at R5 and full maturity stages to determine the 15N and 13C abundance in plant tissues. Seed yield and N content were not affected by different N rates. Symbiotically fixed N accounted for 64% of seed N in treatment N5, whereas fertiliser-derived N dominated seed N in N100, resulting in 58% of seed N. The proportion of soil-derived N in shoot and seed showed no difference between the two N treatments. A similar trend was observed for whole-plant N. The enhanced N2 fixation in N5 significantly increased assimilation of N and C during the seed-filling period compared with N100. Nodule density (nodule number per unit root length) and amount of photosynthetically fixed 13C in roots in N5 were greater than in N100. These results indicate that a greater contribution of N2 fixation to N assimilation during the seed-filling period is likely to meet N demand for maintaining soybean yield when fertiliser N supply is reduced. Greater allocation of photosynthetic C to roots and enhanced nodulation would greatly contribute to the alteration of N acquisition pattern under such condition.
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